Bipolar plates are one of the most important parts of fuel cells which their performance has appreciable effect on cell operation. In this study, Ni-Mo and Ni-Mo-P alloys coatings were prepared on 304 stainless steel substrate by electrodeposition method from citrate-ammonia bath by applying current densities of 30, 60 and 100mA/cm 2 for using in bipolar plates of polymer electrolyte membrane fuel cell. Prepared coatings were uniform and compact.Moreover, molybdenum content of Ni-Mo alloys decreased with increasing current density. On the contrary, composition of the Ni-Mo-P coatings did not change significantly. X-ray diffraction analysis indicated that all coatings were composed of a solid solution which Ni-Mo coatings had nanocrystalline structure with grain size of 47-90? and Ni-Mo-P coatings had amorphous/nanocrystalline structure. In addition, hydrophobicity of substrate was increased with preparing Ni-Mo and Ni-Mo-P coatings. Results of Tafel polarization and electrochemical impedance spectroscopy in simulated solution of fuel cell showed that the solution could not reach the substrate and also polarization resistance, as well as corrosion current density in open circuit potential, was improved compared with substrate. Investigation of potentiostatic results in cathodic condition showed that Ni-Mo-P coatings had almost equal corrosion current density after 4 hours of polarization. On the other hand, corrosion current density was decreased with increasing Mo content of Ni-Mo coatings. Investigation of interfacial contact resistance (ICR) of bare substrate and Ni-Mo coated substrate prepared at 30 mA/cm 2 and Ni-Mo-P coated substrate prepared at 100 mA/cm 2 showed decrease in ICR of substrate with applying the coatings. Moreover, after potentiostatic test of bare 304SS and Ni-Mo-P coated substrate prepared at 100 mA/cm 2 had the maximum and minimum ICR between mentioned samples, respectively. Investigation of performance of bipolar plates in practical conditions of polymer electrolyte membrane fuel cell indicated that the output voltage for substrate decreased faster compared with coated Ni-Mo-P coating obtained at 100mA/cm 2 . In addition, only activation and ohmic polarization regions were observed in diagram of single cell performance with Ni-Mo-P coated bipolar plate. On the contrary, for bare 304SS in addition to mentioned regions, concentration polarization region was also observed and output voltage and power density of cell were extremely decreased in this region. Keywords : bipolar plates, Ni-Mo alloy, Ni-Mo-P alloy, electrodeposition, polymer electrolyte membrane fuel cell
